The present invention relates to video scrambling systems, and more particularly, to such systems that have a high degree of security.
There are various systems of scrambing a video signal for subscription television (STV) services. The only system type currently in widespread use is the so-called "fixed" system. No input from the user or operator of the system is ever needed. A simple fixed system can send the sync (synchronization) signal at a level of, e.g. 10 db less than normal, whereby a receiver unequipped with an unscrambler cannot "lock up" onto the signal, and therefore no intelligible picture can be observed. More complicated fixed systems also scramble the audio signal. While fixed systems differ in how complex the descrambling is, the box that the scheme is built around (that does the descrambling) is something that, once in a person's possession, enables him to receive the programming indefinitely. Regardless of how complicated the box is, a person can obtain and manufacture copies of it. This enables him to illegitimately go into the business and compete with the legitimate source. In fact, it is not even necessary for this person ("pirate") to understand how the box works. All he has to do is be able to manufacture copies of the box. Further, even if no thief sets up in the business of manufacturing boxes, there can be a thriving market in selling stolen boxes which are obtained as a result of burglaries or hijacking.
Another system is called a "keyed" system. A keyed system requires a periodic input, e.g. each month, for proper functioning. This "key" supplied to the box can be an optically coded piece of paper, a magnetically encoded card, or a sequence of numbers punched on a key pad. The key is different each month and the key for one subscriber is different than the key for all others. Without the key insertion, a receiver enable circuit will not produce the proper digital code stream for descrambling. One type of scrambling suitable for use with a keyed system is a random inversion method. This method changes the sign of the modulation sense of the TV signal every line on a pseudorandom basis. A pseudorandom bit stream matching the one which modulates the transmitted signal is generated by the enable circuit of the receiver upon insertion of the proper key. This is used to unscramble the received signal. By its very nature the code can be changed periodically and that is a common feature of all keyed systems. Another method that is suitable for a keyed system, is a line permutation system in which, at the transmit end, a group of lines are stored and sent out in pseudorandom order. A code is necessary to put them back in the right order and that code is generated in the enable circuit by the proper key being given to the descrambler box. Yet another kind of method suitable for a keyed system is one in which each of the TV lines is broken at a pseudorandom point in the middle of the line and the two portions of the line are now sent out in opposite order. The breakpoint of the line is at a different point for each line and again the key for the code is necessary for the descrambler to be able to know where that breakpoint is. There are some problems with this latter method in that the breakpoint tends to show up as an artifact in the received picture. For any of these keyed systems, the pirate may be able to obtain a key and distribute to his customers. This can be done at a relatively slow pace, since the legitimate provider must start distribution of the key to legitimate customers at least a few days before the key changes.
It is therefore desirable to provide a keyed scrambling system for a video signal in a manner that provides a high degree of security against unauthorized descrambling.